1. Field of the Invention
The present invention relates generally to optical apparatuses for processing image data with a laser beam, and more specifically, to an optical apparatus having a laser beam diameter variable according to image data.
2. Description of the Related Art
An example of an optical apparatus of interest to the present invention is disclosed in Japanese Patent Laying Open No. 61-277260 or Japanese Patent Laying Open No. 2-72979. According to these documents, the spot diameter of a laser beam is changed for improving the tone of an image with a reduced number of pixels. The technique can control the beam diameter on a one-by-one pixel basis. As a result, an image nominally having a pixel density of 400 DPI can apparently correspond to an image produced by a printer having a resolution of 800 DPI class. In addition, the memory of the printer is saved.
The memory saving will be described. For a page memory of size A4, for example, a usual laser printer requires about 1.8 MB memory capacity at an image density of 400 DPI. As the pixel density doubles to 800 DPI, the necessary page memory will be four times as large, in other words, about a capacity of 7.2 MB. Prices of memories have fallen in recent years, but a large amount of memories as such increases load on a central processing unit (CPU). As a result, the processing speed of the controller of the printer slows down.
Meanwhile, for a laser printer having a variable beam diameter, it is a different story. If, for example, the beam diameter is controlled in 8 (=2.sup.3) steps, the memory capacity needs only be about three times as large, i.e. 5.4 MB. Such control results in an image having a tone of 800 DPI class, and the processing speed increases as compared to the case of simply increasing resolution.
In a printer having a variable laser beam diameter as described-above, the peak value of Gaussian distribution of a beam changes when the diameter of the beam is altered. Accordingly, in the electrophotographic system, the quantity of light per unit area of the beam is changed. As a result, the amount of charge removed per unit area is changed depending upon the diameter of the beam, and the amount of the sticking toner per unit area will change as a result. More specifically, when the diameter of a beam on an image is changed, the larger the beam diameter is, thinner will be sticking of toner.
Therefore, when a bold line is reproduced with a large diameter beam, the bold line is reproduced onto paper in low density.
Such a printer with a variable laser beam diameter as described above defines a timing for writing an image with an SOS (Start of Scan) sensor. In the laser printer, detection by the SOS sensor is generally not performed until the quantity of light from laser diode emitting a laser beam is stable. Therefore, as disclosed in U.S. Pat. No. 4,264,120, for example, a forced light emitting timing for a laser diode is set earlier.
As disclosed in the above U.S. patent, however, even if SOS detection is performed after stabilizing the quantity of light emitted from the laser diode, in a laser printer which reproduces half tone images by changing the beam diameter, changing the beam diameter on the SOS sensor can shift the starting position of writing an image as a result.
This will be described in conjunction with FIGS. 15a-15c. FIGS. 15a-15c are representations for use in illustration of (a) the positional relation between SOS sensor 17 defining a writing timing for an image in a main scanning direction and a laser beam coming therein, (b) the relation between waveforms of the output of the SOS sensor and threshold levels for producing an image writing position timing, as well as (c) resultant starting positions of writing an image. In FIG. 15, the dotted line corresponds to a large beam diameter, while the solid line corresponds to a small beam diameter. The image writing starting positions are shifted from each other between the larger beam diameter and smaller beam diameter as illustrated at (c). As a result, the picture quality degrades.